The present invention relates to a liquid crystal display device of an active matrix type for displaying image data and character data received from OA equipment or the like, and to the structure of an n-channel thin-film transistor used for this device.
A thin-film transistor (hereinafter abbreviated as TFT) has heretofore been used for driving liquid crystals for each pixel in a panel of a liquid crystal display device of the direct-viewing type. A simple switching element suffices for the performance of the TFT and, hence, amorphous silicon has been used to form the semiconductor thin film. On the other hand, a liquid crystal display device of the projection type requires a high degree of brightness, and the TFT must be realized in a small size to increase the transmission factor. However, it was not allowed to decrease the size of the TFT formed by amorphous silicon, since its current driving ability was so small. Therefore, so-called high-temperature polysilicon has been developed, featuring an increased current driving ability by using quartz glass as a substrate and by polycrystallizing amorphous silicon at a temperature as high as 900.degree. C. or higher.
However, quartz glass is very expensive and drives up the cost of production. Therefore, so-called low-temperature polysilicon has been developed by using an inexpensive glass substrate and polycrystallizing amorphous silicon by irradiation with a laser beam.
In recent years, strikingly improved performance has been exhibited by the TFT formed by using low-temperature polysilicon. Under such circumstances, there is a tendency to utilize the TFT not only as a switching element for driving the pixels of a panel in a liquid crystal display device, but also for the peripheral drive circuits in the liquid crystal display device. Moreover, a liquid crystal display device, such as a system-in-display, is emerging, being furnished with a memory function, as well as various functions of a CPU, interface, I/O and input by pen by using TFTs. In these cases, the role played by the TFT is not limited only to that of a simple switching element; i.e., performance and reliability are required by taking the logic circuits into account.
When the TFT is used as a logic element, eight kinds of voltage patterns will be applied to the three terminals of the gate, source and drain, as tabulated below, wherein "H" denotes a high level and "L" denotes a low level.
______________________________________ Patterns 1 2 3 4 5 6 7 8 ______________________________________ electrode Gate H L H L H L L H Source H H L L L H L H Drain L L H H L H L H ______________________________________
So far, a TFT has been used for driving a liquid crystal pixel, and the above-mentioned patterns 1 to 4 have been exclusively used, i.e., relations of a potential difference across the source and the drain have been exclusively used. When a potential difference develops across the source and the drain, a high electric field is established in the TFT, and a carrier having abnormally high energy (hereinafter referred to as hot carrier) is generated. The hot carrier that is injected into the gate oxide film causes a problem of deterioration in the characteristics of a TFT.
It has heretofore been attempted to solve the problem of a hot carrier that is generated when a high electric field is applied across the source and the drain. As a means for solving this problem, there have been proposed a lightly doped drain (LDD) structure and a double drain structure as disclosed in "Submicron Device 2", by Mitsumasa Koyanagi, Maruzen Co., 1995, p. 187. According to these structures, a high electric field applied across the source and the drain is relaxed to prevent the generation of a hot carrier. These structures involve the case when a single crystal is used as a semiconductor. The same, however, also holds true even in the case of a TFT.